Crude oil, condensate and water produced from wells are typically stored in storage tanks located at or near the well sites. The storage tanks provide temporary storage, as the wells may produce at a slow or uneven rate. Periodically, when a sufficient quantity of liquids have been produced and stored in the storage tank, the liquids are discharged from the tank.
One such method of discharging occurs when a tanker truck visits the well or production site. The truck driver manually connects the storage tank to the truck tanker and discharges the tank contents into the tanker. Another method involves automatically discharging, via a pump, the storage tank into a pipeline such as a sales line, which in turn is connected to a larger pipeline or tanker truck.
For example, one common method is to use, a LACT (Lease Automatic Custody Transfer) unit to automatically discharge and sell the liquids. Most LACT units meter the liquids that are sold so that an accurate measurement is obtained. The TACT unit also operates one or more pumps to move the liquids from the tank to the sales line or tanker truck. The LACT unit operates automatically, with no human intervention needed.
The liquid often contains gas in liquid or vapor form. Gas flashes off of the liquid and rises to the top of the tank. This gas is considered a valuable product and is removed from the tank by separate gas processing equipment for sale.
As liquid is removed from the tank, the pressure of the gas above the liquid decreases due to the increase in gas volume. If the pressure of the gas falls too much, a relief valve on the tank opens to admit atmospheric air into the tank. This is done to prevent the walls and exterior shell of the storage tank from collapsing due to a pressure differential between the outside and inside. The storage tank walls, ceiling and floor are thin and cannot withstand a significant pressure differential.
While admitting air saves the tank from collapse, it has detrimental effects on the downstream gas processing equipment, which includes pipelines, compressors, separation and processing equipment. Air of course contains oxygen and this oxygen causes corrosion and other safety concerns in the downstream equipment.
The same problem arises with water storage tanks. Many wells produce water (typically saltwater), in addition to oil and condensates. Much of the water is separated from the oil at the well site. The water is then stored in a tank that is separate from the oil storage tank. As with the oil, gas flashes off the water and is collected for processing and disposal. A pump removes the water from the storage tank and delivers the water to a pipeline truce, for eventual disposal down a disposal well. The pump can remove the water from the tank too fast, resulting in an inflow of atmospheric air into the tank. This air enters the downstream gas processing equipment.
Thus, it is desired to minimize the admission of air into downstream equipment and in particular into the gas stream or gas circuit of the transfer system.